Theil's index, often referred to simply as "Theil's," is a statistical measure that is used to assess economic inequality or income inequality within a population. Named after the Dutch econometrician Henri Theil, this index is part of a family of generalized entropy inequality measures, and it provides a way to quantify how income is distributed among individuals or groups within a society.
Definition and Components
Theil's index is calculated based on the concept of entropy, which in this context refers to the unpredictability or disorder in the distribution of income. The index can take on different forms, with the two most common being Theil's T and Theil's L. Both forms measure inequality but differ slightly in their sensitivity to changes in different parts of the income distribution.
- Theil's T (GE(1)): This version of the index is more sensitive to changes in the upper end of the income distribution, meaning it gives more weight to the income of the richer individuals.
- Theil's L (GE(0)): Also known as the mean log deviation, this version is more sensitive to changes at the lower end, thus giving more weight to the income of the poorer individuals.
Calculation
The calculation of Theil's index involves the following steps:
- Income Data Collection: Gather data on the income of all individuals or households in the target population.
- Normalization: Normalize these incomes by dividing each individual's income by the mean income of the population.
- Entropy Calculation: Compute the index using logarithmic functions to measure entropy based on the normalized incomes.
Interpretation
Theil's index results in a value that ranges from zero to infinity:
- A value of 0 indicates perfect equality, where everyone has the same income.
- Higher values indicate greater inequality, with more income concentrated among fewer individuals.
Applications
Theil's index is widely used in economics and social sciences to:
- Analyze income distribution within countries or regions.
- Compare inequality levels over time or between different populations.
- Evaluate the impact of economic policies or social programs on income distribution.
Advantages and Limitations
Advantages:
- Theil's index provides a detailed picture of income distribution and is sensitive to changes across different income levels.
- It can be decomposed to analyze inequality within and between groups.
Limitations:
- It requires detailed income data, which may not always be available.
- The interpretation of the index can be complex, especially for non-specialists.
In summary, Theil's index is a valuable tool for understanding economic inequality, helping policymakers and researchers assess the effectiveness of interventions aimed at reducing income disparities.








